Future research

Understanding the relationship and interactions of the conditions of oxalic extraction from a food are important, as this relates to the bioavailability of oxalates in the food system and the human digestion system. This could be different for different types of food groups, for example high starch containing vegetables (eg. taro) compared to berry fruits. Further studies to investigate a wide range different types of food groups should be conducted to gain a clear understanding of the occurrence of oxalate in common vegetables and its bioavailability when cooked and processed.

There is evidence to suggest the fibre content in food has an influence on the distribution of the oxalic content. Is this directly related to the fibre in food binding to key minerals, such as calcium or does the fibre bind directly to the oxalic acid? This warrants

investigation and the implications on whether to supplement fibre in a healthy diet or not. This study has highlighted the need to be vigilant and aware, when new and innovative ways of consuming existing, new and rediscovered foods that may have anti-nutritive factors in them. These could become a new health risk following increased consumption of a food when previously that particular food posed not risk. For example, Martínez- Hernández et al. (2013) vacuum boiled and sous vide broccoli and Shyamala et al. (2005) added leafy vegetable powder to heated oils. It would be useful to investigate potential future risks and to identify them.

There is a need to continue to monitor the uptake of new and re-discovered vegetables and fruits that could cause potential harm. For example, Ruan et al., (2013) reported the total oxalate content of many foods commonly available in Southern China. Many of the foods they investigated had previously not been analysed. It is unfortunate that they did not analyse the soluble oxalate content as this is the most important fraction in foods. Developing future strategies that can be easily adopted and understood by consumers or industry to reduce oxalate levels in food products that have a potential risk. This is an approach that has previously been ignored.

Another area of future research is the development of a low oxalate containing green leafy vegetable by selective breeding or gene manipulation. Morris et al. (2007) demonstrated

124 that a single gene in Medicago trunatula regulated the deposit of calcium oxalate in leaf tissue and this would significantly affect the calcium bioavailability. Approaching this differently, a similar gene manipulation could be used to produce a green leafy vegetable with changed ratios of soluble to total oxalic acid or an overall lower total oxalic acid amount. Traditional germplasm selection for lower oxalic acid levels has been

investigated for spinach in the USA (Mou, 2008) by the USDA. Perhaps similar selection for other commonly used green leafy vegetable needs to be considered.

References

Getting, J. E., Gregoire, J. R., Phul, A. & Kasten, M. J. (2013). Oxalate nephropathy due to ‘juicing’: case report and review. American Journal of Medicine, 126(9), 768-772. Kelsay, J. L., & Prather, E. S. (1983). Mineral balances of human subjects consuming spinach

in a low-fiber diet and in a diet containing fruits and vegetables. American Journal of

Clinical Nutrition, 38(1), 12–19.

Makkapati, S., D’Agati, V. D., & Balsam, L. (2018). “Green smoothie cleanse” causing acute oxalate nephropathy. American Journal of Kidney Diseases, 71(2), 281–286.

Martínez-Hernández, G. B., Artés-Hernández, F., Colares-Souza, F., Gómez, P. A., García- Gómez, P., & Artés, F. (2013). Innovative cooking techniques for improving the overall quality of a Kailan-hybrid broccoli. Food and Bioprocess Technology, 6(8), 2135–2149.

Morris, J., Nakata, P. A., McConn, M., Brock, A., & Hirschi, K. D. (2007). Increased calcium bioavailability in mice fed genetically engineered plants lacking calcium oxalate.

Plant Molecular Biology, 64(5), 613–618.

Mou, B. (2008). Evaluation of oxalate concentration in the U.S. spinach germplasm collection. HortScience, 43(6), 1690–1693.

Ruan, Q. Y., Zheng, X. Q., Chen, B. L., Xiao, Y., Peng, X. X., Leung, D. W. M., & Liu, E. E. (2013). Determination of total oxalate contents of a great variety of foods

commonly available in Southern China using an oxalate oxidase prepared from wheat bran. Journal of Food Composition and Analysis, 32(1), 6–11.

Shyamala, B. N., Gupta, S., Lakshmi, A. J., & Prakash, J. (2005). Leafy vegetable extracts - Antioxidant activity and effect on storage stability of heated oils. Innovative Food

Southgate, D. A. T. (1987). Minerals, trace elements, and potential hazards. American Journal

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Appendix A

Research outline

4 Investigate and optimise the extraction condition of oxalic acid from a green juice

5 Five different green juices chosen, to be made and compositional and oxalic acid comparisons made.

6 A macerating juicer and high speed blender used to compare two types of physical juicing

8 Reducing oxalic acid by soaking the raw materials used to make the juice.

7 The use of a food grade Ca ion source to bind oxalic acid.

 Some green juices are high in soluble oxalate

 The type of juicer used to produce the juice can vary the amount of oxalate  The use of CaCl to lower soluble oxalate is effective either by direct addition

or indirectly by soaking in a solution

The making of green juices

strategies of oxalic acid reduction investigated

Appendix B

Pictures of domestic juicers

C.1 High speed blender

128

Appendix C

Pictures of green juice

D.1 Green juice made from a high speed blender and masticating juicer

High speed blender Masticating juicer - juice Masticating juicer - pulp

Appendix D

List of publications from this thesis